Shape memory alloy motor

ABSTRACT

The invention discloses a novel and efficient drive mechanism for use in a variety of applications. This novel mechanism replaces traditional stepper motors or another analogous art with shape memory alloys. The drive mechanism so disclosed provides substantial operational benefits over conventional motors and other such traditional drive mechanisms that would be used in similar applications. The drive mechanism includes a high gear-ratio worm gear and worm drive which further provides precise, controlled movement of an output shaft. In the illustrative use elaborated upon herein, the motor is used to drive a photovoltaic panel so that the panel may remain in appropriate alignment with the sun throughout the day.

This application is a divisional application from U.S. application Ser.No. 12/583,464, which is a divisional of U.S. application Ser. No.11/236,695, and claims priority from the foregoing applications.

FIELD OF THE INVENTION AND RELATED ART

This invention relates to motor assemblies incorporating shape memoryalloys, also known as SMAs. Drive mechanisms incorporating shape memoryalloys are known in the art. These mechanisms utilize the shape alteringproperties of SMAs to effect required mechanical action.

SMA materials are particularly useful as they have the furthercapability of returning to their original pre-determined shape once theapplication of heat or electrical current is discontinued and the heatdissipates.

As an example, in a conventional solar tracking system, step motors areoften used to drive the motion of solar collectors. In addition, thereexist other, more passive, methods, such as the heating of fluids toshift the center of gravity of a rotating mount. On the whole, thesetechniques tend to be bulky, heavy, expensive, or unreliable, and mayuse a substantial amount of power. The present invention discloses amore efficient and precise drive mechanism which utilizes a shape memoryalloy drive mechanism for motive power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of the tracker assembly with photovoltaic paneland stand.

FIG. 2 is a side view of the tracker assembly.

FIG. 3 is a front view of the tracker assembly.

FIG. 4 is a partial bottom view of the tracker assembly showing thedrive assembly.

FIG. 5 is a schematic of the sensor circuit for controlling forwardmovement of the device.

SUMMARY AND OBJECTS OF THE INVENTION

The present invention provides a motor driven by shape memory alloys foruse in a variety of applications. Shape memory alloys, also known asSMAs or “smart materials”, have the capability of altering their shapeupon the application of heat or electrical current.

This invention provides a modular design, much like a step motor; it canbe coupled with varied controls, including sensors or programmablecontrollers, and varied outputs depending on the required task. Thecurrent design is also unique in its ability to precisely andefficiently move relatively large masses.

The use of SMAs as actuators reduces the size and cost of the drivemechanism while maintaining precision and efficiency. Accordingly, it isan object of the invention to present a drive mechanism incorporatingSMAs that may be used in a variety of applications, under anycircumstances, that require a precise and efficient drive mechanism.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 3, the drive mechanism disclosed includes at least oneshape memory alloy actuator 15, 25, a spring 17, and at least twounidirectional bearings 12. The mechanism also includes a worm drivegear assembly comprising a worm gear 7, worm drive 6, drive shaft 5 andoutput shaft 4. The primary embodiment of the present invention is shownin conjunction with a solar tracking device. Use of the drive mechanismin conjunction with a solar tracking device is merely illustrative inorder to demonstrate the basic mechanics of operation. The drivemechanism may be used in a variety of applications.

Referring now to FIGS. 1, 2 and 3, the solar tracking device itself maybe categorized as having three primary components: a tracking assembly1, a photovoltaic panel 2, and a base platform 3. The photovoltaic panel2 is disposed at the end of a rotatable output shaft 4, which in turn isdriven in a forward motion by the forward motion drive assembly and inreverse by the disengagement assembly. The photovoltaic panel 2 and thetracking assembly, as a unit, are mounted on an adjustable stand thatsupports a base platform 3 that permits the adjustment and fixation ofthe zenith angle of the assembly.

The forward motion drive assembly shown in FIG. 4 includes a drive shaft5 that engages an output shaft 4 through a coupled worm drive 6 and wormgear 7. The output shaft 4 is connected to the photovoltaic panel 2 andis supported by a pair of ball bearings 8 which support the weight ofthe output shaft 4 and the photovoltaic panel 2 while allowing theoutput shaft 4 to swivel. The output shaft 4 is engaged in twolocations, namely, by the worm gear 7 mounted axially on the base of theshaft 4 and by a cable 9 attached to a return spring 10. An output shaftstopper 11 is also disposed on the output shaft 4 in order to preventthe output shaft 4, during return motion, from rotating beyond thepredetermined start position.

The worm gear assembly is responsible for the movement of the outputshaft 4 in the forward direction. In the northern hemisphere, a forwarddirection is from east to south to west. The worm gear assembly consistsof the worm drive 6 and the worm gear 7. The worm drive 6 mounted on thedrive shaft 5 engages the worm gear 7. The drive shaft 5 is supported bytwo unidirectional bearings termed backlash clutches 12. In thepreferred embodiment, there is at least one such unidirectional bearingso as to prevent backlash while supporting the weight of the drive shaft5. A second unidirectional bearing, known as the drive clutch 14, isalso mounted on the drive shaft 5.

A forward actuator wire 15 is provided with one end of said wireattached to the drive clutch 14 at an attachment point 14 a and theother end attached to a fixed mount, in this case the forward mountingplate 16. The forward actuator wire 15 is composed of a shape memoryalloy (SMA). Although more than one type of SMA may be used, the mosteffective SMA in this embodiment would be a nickel titanium alloy. Aswill be appreciated by those skilled in the pertinent art, SMAs composedof other materials (e.g. copper zinc aluminum alloys) may be bettersuited for other applications, depending on the particular requirementsof the application. The forward actuator wire 15 is positioned such thata contraction of the wire causes a rotation in the drive clutch 14. Theforward actuator wire 15 is opposed by the forward spring 17 which isattached to a fixed mount 16 at one end and at attachment point 14 b onthe opposite end.

The entire forward motion drive assembly is mounted on one end of arocking drive platform 18 which is pivotably connected to a locking arm19. The drive platform 18 is further supported near its center by aplatform pivot bracket 20. The platform pivot bracket having a portiondefining a hole in which a platform pivot 20 a is mounted.

The locking arm 19 is provided with locking arm rollers 21 in contactwith the fixed base platform 3 which permit the locking arm 19, when notlocked into place, to remove the lock and pivot the drive platform 18around the platform pivot 20 a. The Locking Arm 19 is pivotable aboutthe arm pivot 22. The locking arm 19 is held by two symmetricalengagement springs 23 that generate a single force which performsmultiple functions. The force generated by the springs 23 holds thelocking arm rollers 21 down against the base platform 3 while forcingthe gears 6 and 7 together by pulling the locking arm 19 into the lockedposition between the drive platform 18 and the base platform 3. Armstoppers 24 are provided so as to limit the angle of rotation of thelocking arm 19 in either direction. The drive platform 18 is pivotablyconnected to the platform pivot bracket 20 which supports the driveplatform 18 while allowing movement around the platform pivot 20 a whenthe gears 6, 7 are to be disengaged or re-engaged.

A disengagement actuator 25, composed of an SMA wire, is attached at oneend to the drive platform 18, or a mounting plate 26 attached to saiddrive platform, and at the other to a point on the locking arm 19, suchthat a contraction of the disengagement actuator 25 would rotate thelocking arm 19 and stretch the engagement springs 23 thereby disengagingthe gears 6, 7. A return spring 10 is provided having one end attachedto a point on the fixed base platform 3 and the other end attached tothe return spring cable 9. The return spring cable 9 stretches from thereturn spring 10, through the return spring pulley 27, and is attachedto the output shaft 4.

The movement of the actuators is controlled through the use of an analogsensing circuit 30. Referring now to FIG. 5, in the primary embodiment,different phototransistors 31 are used to activate separate parts of thecircuit in order to control the different directions of motion. A555-timer 32 is provided to control another transistor, the P-MOSFET,that allows current to flow from the battery 33 to the actuation wires15, 25.

When electric current is applied to the forward actuator 15, theactuator contracts to apply torque to the drive clutch 14. Accordingly,the drive clutch 14 is rotated in the “slip” direction by the forwardactuator 15. The forward spring 17 torques the drive clutch 14 in theopposite direction as the forward actuator 15 cools. With each cycle ofthe forward actuator 15, the drive shaft 5 and worm 6 are rotated whichresults in the turning of the worm gear 7 and output shaft 4.

As the photovoltaic panel 2 is advanced throughout the day, the returnspring 10 is slowly stretched, providing an opposing force to theforward motion while storing mechanical energy to be used later in thereturn motion. The torque from the return spring 10 on the output shaft4 is in the opposite direction of the torque supplied by the worm drive6. When the output shaft 4 (which holds the photovoltaic panel 2) issignaled to return to the start position, the worm 6 and worm gear 7 areseparated so that the output shaft 4 is no longer locked in place. Oncethe output shaft 4 is free to swivel on the support bearings 8, thereturn spring 10 rotates the shaft 4 in the reverse direction untilblocked by a mechanical stopper 11, resetting the photovoltaic panel 2to the sunrise position. The drive platform 18 can pivot away todisengage the gears 6, 7 and then return for re-engagement. Under normaloperation, this happens once per day. While engaged, the drive platform18 is locked into place with a rigid locking arm 19. After the panel 2has completed its rotation with the sun for the day, the unlocking isaccomplished with a disengagement actuation wire 25 that contracts topull on the locking arm 19, working against the pair of engagementsprings 23 to remove the lock and pivot the drive platform 18 away fromthe worm gear 7 thereby disengaging the worm drive 6 and the worm gear7. As the disengagement actuator 25 cools, the engagement springs 23pull the locking arm 19 back into the locked position.

The ratio between the worm 6 and worm gear 7 may be specificallyselected based upon the intended use of the motor, thereby allowing theassembly to be modified in order to maximize efficiency based uponparticular operational conditions. With a low gear ratio, the forwardactuator 15 must pull with more force, but over a smaller distance. Witha higher ratio, the forward actuator 15 is given a mechanical advantage,and can take more precise (though more frequent) steps. This inversebalance between force and displacement can be fine-tuned by adjustingthe leverage in a number of different places in the system. For example,the lengths of the pivoting drive platform 18 and locking arm 19 can bevaried. Also, the diameters of the drive clutch 14 and output shaft 4act as lever arms for the forward actuator 15 and return spring 10respectively. Likewise, the forward actuator 15 might be shorter with alarger diameter to provide a larger force over a smaller distance, or itmight be longer and thinner for a smaller force over a greater distance.With a higher gear ratio, the potential effects of backlash are reduced,thereby minimizing any wasted motion in each forward cycle. For example,with a given output angle per cycle, a higher gear 6, 7 ratio willrequire a greater angle of movement in the drive shaft 5. As the driveclutch 14 is rotated through a larger angle, a smaller portion of eachcycle is lost to play. By using a worm 6 and worm gear 7, not only arehigh ratios easily achieved, but the output shaft 4 is also effectivelylocked in place from external forces on the photovoltaic panel 2 such aswind or vibration.

The use of a motor assembly with shape memory alloys reduces the sizeand cost of the drive mechanism. Specifically, the use of a nickeltitanium alloy in the shape of a wire 15, 25 acts as a mechanicalmuscle, with one “contracting” stroke and one “stretching” stroke ineach cycle. The contraction is accomplished when the actuator is heatedabove the threshold temperature. Heat can be applied to the wire in anynumber of ways, but in this device, an electric current is passeddirectly through the resistive actuator in order to raise itstemperature. When the current stops, the actuator cools and stretchesback to its original length under some opposing force. In a preferredembodiment, each stroke causes a displacement of 3 to 4% of the lengthof the wire, and the cycle can be reliably repeated millions of times.It will be noted that a greater displacement may be achieved byutilizing a SMA component in the shape of a spring but such anapplication would also serve to reduce the overall efficiency of thesystem.

The drive assembly was designed to operate such that the fastercontraction stroke of the forward actuator 15 moves the drive clutch 14in the slip direction, while the slower relaxing stroke moves the driveclutch 14 in the drive direction. Therefore, the forward spring 17 isproviding the force that actually moves the photovoltaic panel 2 againstthe force of the return spring 10 (and any external forces such aswind), so the forward actuator 15 is working against the forward spring17 alone. This provides two important benefits in the preferredembodiment. First, by utilizing the slower, cooling stroke to move thephotovoltaic panel, dynamic effects of acceleration, momentum, andinertia are minimized. Specifically, upon crossing the temperaturethreshold, the actuator 15 contracts with a quick jerking motion,whereas the cooling stroke is slow and controlled. Second, by isolatingthe contraction stroke against the return spring 10, the dynamic forceprofile of each stroke remains very consistent for the actuator 15,enhancing the reliability and lifespan of the actuator 15.

Referring now to FIG. 5, the simplest method of controlling the movementof the motor in this particular embodiment is through an analog sensingcircuit 30. There must be at least one sensor for each direction ofmotion. In this design, different phototransistors 31 are used to turnon separate parts of the circuit 30 to control the different directionsof motion. Multiple phototransistors can be used for each direction ofmotion by connecting them in parallel. The light-sensitivity of thesephototransistors can be adjusted by changing the value of the resistorR3. Once a phototransistor activates the circuit, a 555-timer 32controls another transistor that allows current to flow from a battery33 to one of the actuation wires 15 or 25. The timer is used to converta continuous “on” signal from the phototransistor to a cyclic “on-off”signal, allowing the actuator to heat and cool repeatedly. The necessary“on-time” and “off-time” are determined by the size of the actuationwire and power source, and can be adjusted to the correct duration bythe values of two resistors R1, R2 and one capacitor C that areconnected to the timer 32. Also, the timer 32 itself receives no powerwithout a signal from one of the phototransistors 31 (because of a breakin the ground line) in order to minimize overall power consumption. Inthe primary embodiment, the current is drawn from a battery 33 that hasbeen charged by the photovoltaic panel 2, but it will be recognized thatthe power could also come from a photovoltaic panel directly, a bank ofcapacitors, an alternating current line, or any other electric powersupply.

When the timer 32 is not grounded, it consumes no power, and theP-MOSFET breaks the circuit to the actuator 15 so that no movement canhappen. If F1 or F2, or both, receive light, then the timer 32 becomesgrounded and begins to count (by charging and discharging the capacitorC through R1 and R2). Once the timer 32 has completed the first periodof its two-step cycle, it toggles the output signal to close theP-MOSFET, allowing current to flow from the battery 33 (or photovoltaicpanel, capacitor bank, AC, or any other power supply), through theP-MOSFET, then the actuator, to the ground. This heats the actuator,causing it to contract. When the timer has finished the second period ofits cycle, the output of the timer is toggled once again, so theP-MOSFET breaks the circuit, allowing the actuator to cool and stretch.If the previous cycle did not rotate the output shaft 4 far enough, thenthe forward sensors will still be in the light, and the cycle continuesuntil they are shaded. When all the sensors are shaded, the timer groundis broken so no further action is taken. The backward part of thecircuit works exactly the same way. It is identical in form, but mayhave different values for C, R1, or R2 (for timing purposes), hasphysically different sensor positions and orientations, and a distinctactuator 25 which may differ in size from the forward actuator 15. Itwill be recognized by those in the art that other controller types,including digital circuits, may be used to accomplish the foregoingtasks.

Although the shape memory drive mechanism has thus far been described inrelation to a solar tracking device, this device can be used as amodular step-motor for many applications outside of solar power. Theshape memory alloy actuators are small, light-weight, inexpensive,reliable, quiet, and efficient.

The primary embodiment disclosed herein fits the solar power applicationbecause only the forward direction requires precision, while the returnmovement can be taken as a single leap. Also, multiple outputrevolutions are never needed. However, if precise motion were requiredin both directions, the same principle could be used, but with a gearshifting, relying upon more than one worm gear assembly, rather thandisengaging, allowing the same “forward drive” to work in the oppositedirection as well.

The sensing circuit 30 discussed in the photovoltaic application can bereplaced with a programmable microprocessor. This inexpensive controlcan be very robust, and can work with a variety of inputs, such asprograms or other sensors, to execute any number of different tasks. Forincreased speed, the drive shaft 5 can be fitted with multiple driveclutches, each with its own actuator/spring pair, working in a sequencedwave like pistons in a combustion engine. For increased strength, theforward actuator 15 size can be increased. Thicker actuators will pullwith more force, and longer actuators will pull a greater distance.Therefore, longer actuators can be used for a greater angle of rotationin the drive shaft 5, or they can be mounted on a drive clutch with alarger diameter to turn the shaft 4 over the same angle, but withgreater force. When using a worm 6 and worm gear 7 with a high gearratio, extremely precise movements can be obtained, with steps of afraction of a degree, packaged in a small modular case much like atraditional electromagnetic step motor. It will also be noted that therelative positions of the actuator(s) and spring(s) could be reversedallowing the motor to drive with the faster contracting stroke ratherthan as operated in the embodiments set forth thus far.

The driving mechanism can be used in other applications besides solartracking. It can be used in place of a motor for any low-speed orlight-duty application. This mechanism can be customized to track manyphysical phenomena (that can be sensed by a sensor or a detector).Examples include: light source (different spectrum within), radiosignals, motion, wind, sound, magnetic field, chemical and biologicalagents, etc on earth and possibly in outer space.

While the invention has been described in reference to certain preferredembodiments, it will be readily apparent to one of ordinary skill in theart that certain modifications or variations may be made to the systemwithout departing from the scope of invention claimed below anddescribed in the foregoing specification.

1. A drive mechanism comprising: a fixed surface; said fixed surface having a platform pivot portion; a rotatable output shaft supported on said fixed surface by at least one support bearing; a worm gear fixed to one end of said output shaft; a locking arm in rollable contact with said fixed surface; a drive platform in pivotable contact with said locking arm at a first end and a drive mechanism disposed at the other end; said drive platform in pivotable contact with said platform pivot; said drive platform having a locked position and a disengaged position whereby said locked position allows said drive mechanism to remain in mesh with said worm gear and further whereby said disengaged position allows said worm gear to rotate freely; and a means to power said drive mechanism.
 2. The device as in claim 1 wherein said means to power said drive mechanism further comprises a means having at least one nickel titanium alloy actuator and said means being operable absent a source of solar radiation.
 3. The device as in claim 1 further comprising a means to disengage the drive platform from said worm gear. 